(1) Field of the Invention
The present invention relates to the detection and amplification of a low level signal from a piezoelectric sensor. It is intended that this preamplifier provide the gain, frequency response shaping, impedance matching, static charge dissipation, and low noise floor necessary in order that this device serve as the sole component between a piezoelectric sensor and any follow-on circuitry such as an analog-to-digital converter. It is further intended that this invention be suitable for incorporation on the same monolithic integrated circuit as the follow-on circuitry.
This device can be used wherever a piezoelectric sensor requires a high-impedance, low noise interface circuit. The applications for the preamplifier include sonar hydrophones, piezoelectric accelerometers, pressure sensors, and any other piezoelectric sensor which has very small capacitance and thus high impedance. The preamplifier can be incorporated onto a monolithic integrated circuit with an analog-to-digital data interface function, thus achieving a lower total noise than could be obtained by direct connection of the sensor to an analog-to-digital converter. This device can be mounted directly onto or inside a miniature sensor without degrading the sensor's performance.
(2) Description of the Prior Art
A piezoelectric sensor can be thought of as a voltage source in series with a capacitor. In certain applications it is necessary that this sensor be very small. Two such examples are, high frequency beamformed acoustic arrays which require very small, omni-directional acoustic sensors and miniature accelerometers which require small mass. Typical existing preamplifiers are built from discrete electrical components and are frequently larger and more massive than the sensor itself. This results in a degradation of the sensor performance due to signal interaction (i.e. scattering, reflection, loading, etc.) if the preamplifier is located close to the sensor. A sensor connected to a preamplifier that has discrete electrical components is shown in U.S. Pat. No. 4,013,992, Diver's Piezoelectric Microphone with Integral AGC Preamplifier, Larry F. Dewberry et al., inventors. One remedy is to relocate the preamplifier a significant distance from the sensor. A problem with preamplifier relocation arises due to interconnecting cable capacitance which attenuates the sensor signal and thus reduces the sensor signal-to-preamplifier noise ratio. Thus, previously available preamplifiers limit the performance that can be achieved from small sensors.